In automated assembly processes using robots, the robot arm is usually provided with a so-called end effector, or gripper assembly, for picking and placing parts. The gripper assembly generally includes a housing rotatably mounted to the end of the robot arm and a pair of fingers depending from the housing and movable relative to one another by appropriate actuators contained in the housing. In one known form of gripper assembly, the actuators include D.C. servo-motors which rotate threaded rods in one direction or another for displacing the fingers either toward or away from one another. With D.C. servo-motor actuator systems, the gripping force is a function of the the motor output torque, which in turn is a function of the motor input current. This permits the gripping force to be controlled by control of the current supplied to the motor.
While D.C. servo-motor controlled actuator systems have certain advantages when used to drive robot gripper fingers, they also have certain limitations. For instance, they require sophisticated and expensive detectors and the like for monitoring and controlling the positions and gripping forces of the fingers. They employ closed loop feedback control that is time dependent and that absorbs large amounts of the operational time and/or capacity of the computer associated therewith. Additionally, the analog nature of the D.C. servo-motor systems complicates interfacing thereof with computers of the preferred digital type.
The foregoing factors contribute significantly to the "computer overhead" expense, and thus to the overall relatively high cost, of gripper actuating systems employing D.C. servo-motors.